Caption: Coloured scanning electron micrograph (SEM) of Streptococcus salivarius, Gram-positive, coccoid, facultatively anaerobic prokaryote (bacterium). Streptococcus salivarius forms chains of cells which are bound together by the capsular material seen around each cell (shown as surface stipulation in this image). It is the principal commensal bacterium of the oral cavity and a normal inhabitant of the upper respiratory tract in humans. It is the first bacterium that colonizes dental plaque, before being joined by numerous other species of various genera. It creates favourable conditions so other species can begin to colonize. The bacterium colonizes the mouth and upper respiratory tract of humans a few hours after birth, making further exposure to the bacteria harmless. S. salivarius is considered an opportunistic pathogen and when it enters the blood stream it causes septicaemia in people with neutropenia. Magnification: x5,335 when shortest axis printed